1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device. More specifically, the invention relates to a nonvolatile semiconductor memory device, such as a so-called one-time programmable memory, using nonvolatile storage elements to which data can be written only once.
2. Description of the Related Art
Conventionally, one important object of a nonvolatile semiconductor memory device that can hold data even after its power is turned off is to prevent data from being lost due to a malfunction caused when the power is turned off or it is turned on again. The object is more important particularly to a one-time programmable memory to which data can be written only once or no data can be rewritten. The object becomes more important to a nonvolatile semiconductor memory device using a fuse element or an anti-fuse element to which data is written using a high-current power supply and a high-voltage power supply, because a write control circuit malfunctions to not only lose data but also break the device (semiconductor elements of the circuit).
The above malfunction of the write control circuit is caused by an unexpected operation thereof. Conventional nonvolatile semiconductor memory devices have employed a so-called protection circuit with a function such as a key and a password to inhibit a write control circuit from operating without executing normal procedures. Such a protection circuit is however complicated in general. For example, a sequential circuit is often used to confirm that normal procedures have been executed in sequence. However, the malfunction of the write control circuit is caused chiefly by an unstable state of a power supply when it is turned off or turned on again. The protection circuit complicates the write control circuit, and the sequential circuit weakens the write control circuit.
Assume here that a protection circuit is configured as follows. The protection circuit is supplied with KEY [2:0] corresponding to three input signals. If KEY [2:0] is first brought into “010” state and the circuit is supplied with a clock CLK, the first key is released. If KEY [2:0] is then brought into “101” state and the circuit is supplied with a clock CLK again, a write operation starts. Such a protection circuit always includes a sequential circuit using a flip-flop. However, the sequential circuit generally becomes unstable immediately after power is turned on. The flip-flop could be brought into an unexpected state by the influence of radiation or the like. The protection circuit therefore fills the role of preventing an unauthorized write operation while the sequential circuit is normally operating; however, it cannot prevent an unauthorized write operation due to a malfunction of the sequential circuit. Adding a power-on reset circuit that monitors the state of a power supply and generates a reset signal, the initial state of the protection circuit can be determined. The protection circuit is thus improved in advantage (accuracy) accordingly. However, the power-on reset circuit is not advantageous to any malfunction due to radiation or its operation is not performed stably.
A nonvolatile memory including an address holding circuit having a reset signal input has been proposed (refer to Jpn. Pat. Appln. KOKAI Publication No. 2003-131951, for example). However, the nonvolatile memory is configured to reliably prevent data from being written to the memory or erased therefrom when a reset signal is generated during the execution of a write sequence or an erase sequence.